line = int(input())
to = line - 1
edge = int(input())
graph = [[0 for i in range(line)]for i in range(line)]
for i in range(edge):
    n, m = map(int, input().split())
    graph[n][m] = 1
level = [0] * line
marked = [False] * line
__________
queue = [0] * line
head = 0
tail = 1
while head < tail and not flag:
    __________
    head += 1
    for j in range(line):
        if j != c and graph[c][j] == 1:
            if j != to and marked[j] == False:
                queue[tail] = j
                tail += 1
                marked[j] = True
                level[j] = level[c]+1
            elif j == to:
                __________
                flag = True
                break
print(level[-1]-1)